Radio frequency identification (RFID) is becoming an important identification technology in applications such as inventory management, security access, personnel identification, factory automation, automotive toll debiting, and vehicle identification, to name just a few. An RFID system utilizes an RFID transmitter-receiver (interrogation) unit (usually referred to as a base station or interrogator) to query an RFID transponder or tag. When located within an operational distance from the interrogation unit, the RFID tag detects the interrogating signal and transmits a response signal containing encoded data back to the receiver unit.
Current developments in the area of RFID tagging and tracking systems have generally evolved into two distinct technology fields, i.e., near-field devices and far-field devices, the latter generally employing higher frequencies and being operable over longer ranges than the former. Both device types employ some architecture common to both. For example, both types of systems employ an interrogator and at least one transponder tag. The interrogator transmits a query signal to the tag and receives data transmitted from the tag. The architecture of the interrogator generally employs a transmitter/receiver (transceiver) and a reader.
The transceiver may include an antenna and an interface with which to transmit and receive signals to and from the tag. The reader provides the interrogation signal and evaluates the received signals to and from the tag. The reader may include a modulator/demodulator (modem) that communicates with the antenna, a coder/decoder (codec) and a controller for communicating with a processor that handles application-specific functions.
The architecture of each tag generally employs an antenna, a rectifier circuit, and a state machine for modulating the transmitted data. A logic circuit represents one example of a state machine that sequentially transitions through a series of predetermined output states in conjunction with applying to the tag circuitry a series of pulses, such as by a clocking circuit.
Typical RFID tags have a patch antenna, a diode detection trigger to initiate the tag response, and transmission circuitry to send the responding data back to the interrogator. Near-field tags typically employ simpler circuits than far-field tags. Transmission circuitry for the near-field tag usually includes the state machine and a transistor device, the latter being used to change the impedance of the patch antenna.
Most of these near-field RFID systems use a technique called back-scattering. The interrogator for a backscatter system transmits an unmodulated, single frequency continuous wave (CW) signal that triggers or “awakens” the transmitter on the tag. In response, the tag modulates the reflection of the received CW signal by changing the impedance of the antenna in the RFID tag in accordance with a code supplied by a state machine to prevent “collision” of data between two or more tags in close proximity.
The RFID tags in a near-field RFID system may or may not have a battery to provide power for returning a signal to the interrogator. Tags without a battery are labeled “passive”, while those with a battery are labeled “active”, with simpler tags typically being the former. Passive tags draw power from an electromagnetic field created by the interrogator signal received by the patch antenna. The independent power source for active tags enables longer communication distances, independent sensing and monitoring, self-initiated transmission, and higher bandwidth than passive tags at the sacrifice of higher cost and size.
In contrast, the tags employed in a far-field RFID system have developed around the use of microprocessors and software algorithms. In operation, the interrogator sends a modulated radio frequency (RF) signal to the RFID tag. The microprocessor embedded in the RFID tag interprets the received signal and determines the need for a response signal, i.e., determines whether or not a response signal is warranted.
The far-field RFID system avoids data collision because the tag will respond to the interrogator only when the tag receives a modulated signal that matches a code stored in the tag and accessed by its microprocessor or state machine. If the received signal matches a predetermined code, the tag “awakens” and transmits the requested data via a radio circuit. The encoding and the transmission operations by the tag may be performed by either a microprocessor or a state machine that performs the carrier modulation. These circuits are fairly intricate and thus expensive due to the use of microprocessors. Because of their power requirements, the far-field RFID system frequently employs active tags.
What is needed is a low cost, far-field RFID tagging and tracking system. Desirable characteristics include RFID tagging and tracking system could utilize low cost and/or passive RFID tags without encountering data collision. Such systems would be advantageous if the RFID tagging and tracking system can be utilized in the tracking of inventory and assets for a wide range of applications including, but not limited, to inventory data collection in a warehouse system, tracking of mobile assets, and security monitoring.